In general, during the development of new motor vehicles and the further development of existing motor vehicles, in particular also commercial vehicles, an aim is to reduce fuel consumption and the emissions of pollutants and CO2. Besides technical optimization of the vehicles, such as the development of low-consumption and low-emission drive motors, efficiency-optimized multi-stage transmissions, tires with low rolling resistance and aerodynamically shaped bodies, truck driver cabins and truck superstructures, there is a further possibility for reducing fuel consumption and pollutant and CO2 emissions, namely by suitable control of the driving mode during the use of the motor vehicles.
For this, it can be provided that while driving under suitable operating conditions, a motor vehicle can be operated temporarily in the so-termed rolling mode or in the so-termed coasting mode. In rolling operation the drive-train remains closed, i.e. the gear engaged in the transmission remains engaged, the transmission ratio set remains set, and the separator clutch remains engaged. Only the fuel supply to the drive motor is cut off. Since in such a case the drive motor changes to drag operation and the drag torque of the drive motor therefore acts as a braking force on the drive wheels, more prolonged rolling operation is in practice only possible when moving along a slightly downhill stretch, driving over the brow of a hill and driving through a depression, when the downhill propulsion force acting on the motor vehicle, at least averaged over time, is to a large extent compensated for by the rolling resistance of the motor vehicle and the braking force of the drive motor.
In contrast, during coasting operation the drive-train is opened, which can be done by disengaging the separator clutch and/or by engaging the neutral setting in the transmission. The drive motor is then for the most part operated in the idling mode, but provided it is not needed for powering safety-relevant and/or comfort-relevant aggregates such as the servo-pump of a servo-steering system or the compressor of an air-conditioning unit, it can also be switched off entirely. Since in that case there is no drive motor drag torque, a longer unpowered propulsion phase or coasting operation is possible more frequently and on various road profiles.
In general however, besides the basic current operating conditions that must be met for the initiation of coasting operation, it is difficult to define, measure and evaluate road-stretch-specific criteria which allow a transition to coasting operation to seem appropriate having regard to economic and comfort-orientated considerations. The current conditions to be met for coasting operation include a running drive motor, a current driving speed higher than a defined minimum speed, the absence of a demand for acceleration (accelerator pedal not actuated and/or no engine torque demand from an activated cruise control), the absence of a deceleration demand (brake pedal not actuated and/or no braking torque demand from an activated cruise control), and if necessary also a current road inclination between a maximum permitted downhill slope and a maximum permitted uphill slope.
Conditions related to this are known, for example, from DE 102 21 701 A1 which describes a method for controlling the driving mode of a motor vehicle, with which a speed adaptation of the drive motor is intended to enable a rapid and largely smooth termination of a coasting operation.
Since fulfilling of the conditions alone is not sufficient for a longer and economically worthwhile coasting operation, at least one further criterion must be fulfilled, which relates to a road section lying directly ahead of the motor vehicle, along which there is a high probability that the vehicle is about to drive.
DE 10 2004 017 115 A1 describes a method for controlling the driving mode of a motor vehicle during the operation of a speed regulation unit, in which with the drive-train closed the motor vehicle is periodically accelerated by the drive motor to an upper speed limit above the nominal speed and then, in rolling or coasting operation, decelerated to a lower speed limit below the nominal speed. When the road slopes down steeply the motor vehicle is slowed down in rolling operation, i.e. with its drive-train closed and the fuel supply to the drive motor cut off. If the road slopes downhill less steeply the motor vehicle is slowed down in coasting operation, i.e. with the drive-train open, and stopping of the drive motor is only provided for if the expected duration of the deceleration phase exceeds a specified length of time or if the motor vehicle is immediately before a steep downhill stretch. For controlling the sequence of acceleration and deceleration phases in a manner optimized for consumption and emissions, topographical parameters should also be taken into account.
In contrast, DE 10 2006 054 327 A1 discloses a method for controlling the driving mode of a motor vehicle, according to which the current vehicle position and the current driving speed of the motor vehicle are noted and processed with reference to data of a digital road map, in particular with road inclinations and speed limits of road sections ahead of the motor vehicle, in order to determine an optimum starting point for initiating a coasting operation. By pre-calculating the rolling speed during coasting operation, the starting point for initiating the coasting operation should be determined such that statutory speed limits associated with road sections laying ahead can be complied with out actuating a brake (service brake or retarder).
Finally, DE 10 2008 023 135 A1 describes a method for controlling the driving mode of a motor vehicle, according to which, during driving the current driving speed of the motor vehicle, vehicle parameters that represent the rolling properties of the vehicle and topographical data of the road stretch are determined continually, and from these the speed profile expected within a road stretch lying ahead while in rolling operation, i.e. with the drive-train closed and the fuel supply to the drive motor cut off, is calculated. Depending on the speed profile concerned relative to a maximum speed and a minimum speed, it is decided whether to begin, block or terminate rolling operation. From the calculated speed profile the brow of a hill or a depression can be recognized and the beginning of rolling operation is determined in such manner that the vehicle can travel over a brow of a hill without its speed falling below the minimum speed and through a depression without exceeding the maximum speed, in each case during rolling operation, and so that the rolling speed at the end of rolling operation corresponds largely to the driving speed at the beginning of rolling operation or to the nominal speed set by an activated speed regulation unit.
However, the methods mentioned above all deal only with partial aspects of the initiation or termination of the rolling or coasting operation of a motor vehicle. Likewise, the definition of the limiting or minimum and maximum speeds used for evaluating the speed profile calculated in each case, is unclear. A further disadvantage of the known methods is that they each provide for comparatively fixed criteria for the initiation and termination of rolling or coasting operation, which make no allowance for the occurrence of abrupt changes of the calculated speed profile that can be caused by signal scatter and signal interference. The result can be that a rolling or coasting operation is started and terminated again several times in brief alternation, which substantially reduces the possible energy saving and impairs the driving comfort.